Various types of fluidic actuators are utilized for converting pressurized fluids such as air or hydraulic fluid to mechanical motion. These actuators include the common piston-cylinder drive in which a piston slides within the chamber of a cylinder and is driven by a differential in fluid pressure across the piston, as in the very common commercially available air cylinder drives and hydraulic rams. Such actuators can have a relatively long stroke but are limited in applied force to the fluid pressure across the piston times the surface area of the piston. Another type of fluidic actuator simulates the action of natural muscle contraction. An elastic tube or bladder is surrounded by a sleeve or sheath of relatively inelastic material, typically braided fibers, and the two ends of the sheath and the central tube can be connected by end fittings to other mechanical structures. When a fluid under pressure, such as air or hydraulic fluid, is introduced into the inner bladder, it expands along its length, forcing the fibers of the surrounding sheath outwardly, drawing the two ends of the actuator closer together and exerting a force on the structures to which the actuator is attached. Because the inner tube or bladder is inflated outwardly along essentially its entire length, the cumulative outward force exerted on the surrounding sheath can be very great, so that very large forces can be applied by the actuator over a relatively small range of travel. Examples of such fluidic actuators are shown in U.S. Pat. Nos. 3,830,519, 4,739,692, 4,751,869, 4,819,547, 4,841,845, 5,014,600, 5,021,064, 5,052,273, 5,185,932, and 5,351,602.
While the forgoing contractile fluidic actuators are well-suited to applications requiring high forces applied over short distances because of their compactness and potential relatively low cost, such actuators have been subject to certain practical problems that have limited their use. One problem stems from the fact that the relatively soft and flexible inner bladder or tube is brought repeatedly into and out of contact with the harder and less resilient fibers of the outer sheath. Over many contraction cycles, the repeated contact between the elastic bladder and the sheath can abrade the material of the bladder, eventually leading to leaks in the bladder and complete failure of the actuator after a relatively short service life. Another difficulty encountered in practice relates to the fittings that are connected to the ends of the sheath. The mechanical connection between the fibers of the sheath and the fittings must withstand the full force applied by the actuator and must be capable of doing so over many contraction cycles. Typically, a fluid coupling is also incorporated into one of the end fittings so that the fluid can be introduced at one end of the actuator rather than at some intermediate position. This fluid coupling fitting must be securely connected to the tube so that the tube will not disengage from the fitting during use, and preferably, it is also connected to the outer sheath to form part of the structural connecting fitting. Conventional crimp type collars have been used to hold the sheath on the fittings, but these may not perform satisfactorily to hold the sheath and fitting together over an extended number of contraction cycles. To use a sufficiently strong and robust connector between the sheath and fitting can significantly increase the total cost of the actuator and add to its bulk.